1. Field of the Invention
Aspects of the present invention generally relate to apparatus and methods of operating a rod-pumped well. Particularly, aspects of the present invention relate to an apparatus for controlling the operation of a rod-pumped well where the apparatus is mounted on a walking beam (or structural member) of a pumping system. More particularly, aspects of the present invention relates to an integrated control apparatus for operating a pumping system and measuring strain on the polished rod.
2. Description of the Related Art
Oil well rod pumping systems sometimes require a method to accurately determine the weight of the fluid in the production tubing during operation. This information is primarily required on wells that “pump-off”, that is wells that do not produce enough fluid to permit them to be pumped continuously. When a well has been pumped off and there is insufficient fluid present in the wellbore at the pump intake, the pump is said to be undergoing “partial filling.” Partial filling is an undesirable condition because it lessons the overall efficiency of the pumping system and may cause system failures over the operating life of the producing well.
Generally, partial filling causes fluid pounding, which can be damaging to various components of the pumping system. Fluid pound is typically caused by the pump not completely filling with fluid on the upstroke. As the downstroke begins, the entire fluid and rod string load moves down through a void until the plunger hits the fluid level in the pump barrel. When the traveling valve opens, the load is suddenly transferred to the tubing, thereby causing a sharp decrease in load. As a result, a shock wave transmits through the pumping system. The shock wave produced may damage the components of the pumping system.
To reduce the occurrence of partial filling, and to produce a well at or near maximum efficiency, a pump off control system is typically used on these wells. A pump-off control system generally includes a controller, a sensor for detecting the weight of the fluid in the production tubing during operation of the pumping system, and a device for measuring the position of the pumping system over each cycle of stroke. Examples of the load measurement devices employed for pump off control include use of load cell based technology installed on the pumping rod or mounted on the walking beam. Generally, these devices interface with the controller to produce information for well analysis. Analysis of this information will provide data relating to the amount of fluid in the wellbore and the accurate detection of fluid pound. The control system will shut the pump down when it determines that the wellbore is partially full or empty, thereby avoiding excess wear on the pumping equipment and also saving energy. The pump-off control system also protects the pumping system in the event of a critical malfunction in the sucker rod string or drive train. The system is turned off when such malfunctions are detected.
A device for measuring strain in the polished rod of a rod-pumped well unit is disclosed in U.S. Pat. No. 3,965,736 issued to Welten, et al. Welten discloses a system utilizing a strain-gage transducer welded to the top flange of the walking beam of an oil well pumping unit. The sensor is welded to the walking beam in order to achieve maximum sensitivity. A cable is used to connect the system to a controller.
More recently, a strain measuring device utilizing an integral clamp-on mechanism is attached to the load-bearing surface of the walking beam or any convenient location as disclosed in U.S. Pat. No. 5,423,224 issued to Paine, which is herein incorporated by reference. This device eliminates the requirement for welding of the load measurement device to the walking beam, thereby allowing for easier installation and maintenance of the device. However, this device, as with the Welten system, requires a cable to connect the transducer to the controller. In FIG. 1., a pump off control system, according to Paine, includes a strain measuring device 1 attached to the walking beam 2 of the pumping system 3. Information from the device 1 is relayed via cable 4 to the controller 6. After processing the information, the controller 6 sends signals to the motor control panel 5 to operate the pumping system 3.
Although the pump off control system shown in FIG. 1 is widely utilized, the pump off control system is difficult to install and maintain. For instance, to install the pump-off control system on an existing pumping system, a controller must be installed near the pumping unit, which, in most cases requires trenching, a pole to mount the controller, and cement to hold this structure in place. In addition, cables must be used to connect the various components of the system to relay information. To accommodate the landscape, the installation of the pump-off control system may be different each time, thereby requiring modification of the installation materials and procedure. Typical installation times per system may exceed several hours and require personnel of varied skill levels. Also, several key areas of this pump off control system require on-going maintenance, such as the cable interconnecting system. Further, the pump off control system may be susceptible to failure due to wear of the cables and the normal maintenance process for the pumping system.
There is a need, therefore, for a pump off control system that offers less complexity to install and that can be easily maintained. There is a further need for a pump-off control unit having an integrated controller and a pump rod load measuring device. Further still, there is a need for a pump-off control unit having an integrated controller and a pump rod load measuring device that transmits a control signal using a cable-less communications system.